1. Field of the Invention
The present invention relates to an apparatus and method for diagnosing a hydraulic variable valve timing mechanism, and in particular to technology for diagnosing a fault in a hydraulic variable valve timing mechanism in which the valve timing is changed by oil pressure.
2. Description of the Related Art
Variable valve timing mechanisms which advance or retard the open-close timing of an intake valve and/or an exhaust valve in a vehicle engine by using oil pressure to change the rotational phase of a cam shaft with respect to a crank shaft have been previously reported (refer to Japanese Unexamined Patent Publication No. 7-233713 and Japanese Unexamined Patent Publication No. 8-246820).
However, with the conventional variable valve timing mechanism mentioned above, when a fault diagnosis is conducted by comparing a target valve timing (target rotational phase) and an actual valve timing (actual rotational phase), various problems can arise, as detailed below.
With a hydraulic variable valve timing mechanism, a stepwise change of the target valve timing results in the actual valve timing gradually approaching the changed target due to a response delay in the oil pressure.
As a result, the problem arises that a comparison of the actual valve timing with the target valve timing immediately following a stepwise change of the target valve timing, produces a misdiagnosed fault based on the existence of a relatively large deviation between the two values.
In particular, if the oil drains out of the hydraulic chamber of the variable valve timing mechanism during the engine is stopped, then the first change of the valve timing following engine start-up will require the hydraulic chamber to be refilled with oil in order to generate the oil pressure necessary for changing the valve timing. Therefore, in relation to the valve timing change command the actual valve timing will change even more slowly than normal, increasing the possibility of a misdiagnosed fault.
Furthermore, in those cases where the actual valve timing is detected as the rotational phase of a cam shaft with respect to a crank shaft, then for example when a construction is used wherein a plate supported on the cam shaft incorporates a plurality of detection portions which are detected by an electromagnetic pickup detector or a Hall element detector, then due to deviations in the positions of the detection portions, deviations develop in the detection results for the rotational phase values at each detection portion even though the valve timing (rotational phase) is actually constant. Consequently the possibility arises that the detected value for the rotational phase may temporarily exceed the fault judgment level resulting in a misdiagnosed fault.